On an EFI system, GRUB can chainload other EFI bootloaders. For example, a Windows installation on GPT can be booted with
chainloader /efi/Microsoft/Boot/bootmgfw.efi

Such a line can currently not be generated with boot-update.

For a case like windows on GPT, a specific type like "win7gpt" or such could be created, but I think it would be nice, if there was an "efi" entry type for boot.conf, where the "kernel" (or some new) option specifies the efi file to load.

As a temporary fix it would be nice, if the deletion of grub-mkconfig in the grub ebuilds was optional.